a) Field of the Invention
The present invention relates to an improved method for producing high transition temperature superconducting ceramic elements of given length, which method is much more rapid and efficient than the existing ones.
In the following disclosure, "element of given length" means a wire, a ribbon, a rod, a ring, a cylinder or any similar article of elongated shape, made up of superconducting ceramic material.
Similarly, "high transition temperature a superconducting ceramic" means any superconducting ceramic material such as, for example, Y Ba.sub.2 Cu.sub.3 O.sub.7.delta. for oxides of Bi, Sr, Ca and Cu, or of Tl, Sr, Ca and Cu, which ceramic material has a high transition temperature Tc greater than the boiling point of liquid nitrogen (77K).
b) Background Of the Invention
High transition temperature superconducting ceramics are heralded as important materials for future high-current-density, low-field applications in electric utility equipment and elsewhere.
To make these ceramics actually useful for practical applications, it is necessary to produce them in the form of wires, ribbons, rods, rings or cylinders.
Such a production has proved up to now to be complex and arduous because of the ceramic nature of these materials which, on the one hand, have a granular structure even after sintering and which, on the other hand, always contain some secondary phases, unwanted impurities or pores, which altogether contribute to limit their high critical current densities.
Recently, techniques have been developed to tentatively overcome these problems. One of these techniques, which has proved so far to be the most efficient, consists in melting and texturing a wire, ribbon or rod of superconducting ceramic precursor material in a fashion which resembles the zone melting technique used in the field of metallurgy for the purification of metal, in order to achieve (1) ordered ceramic grain growth by directional solidification and (2) high purity over most of the length of the ceramic element due to the "natural" property of the impurities to accumulate in the liquid phase (or liquidus) of the melt and thus to concentrate in this melt as it "moves" along the wire, ribbon or rod.
This "melt-texturing" technique and its application to the production of wires of superconducting ceramic material a few centimeters long, is disclosed, by way of sample, by R. L. Meng et al in Nature, 345, 326 to 328g (1990) and by P. J. McGinn et al in Physic, C-161, 198-203 (1989) and Physic, C-165, 480-484 (1989).
In practice, this technique comprises four basic steps which are as follows:
1--forming a wire, ribbon or rod made up of grains of ceramic precursor material; PA1 2--sintering these grains; PA1 3--subjecting the resulting sinter to zone melting by passing the wire, ribbon or rod through a tube furnace having a narrow hot zone whose temperature is above the peritectic decomposition temperature of the ceramic precursor material; and PA1 4--subjecting the obtained melt-textured ceramic element to oxygen-annealing in a separate furnace to adjust the oxygen content of this element to any predetermined value known to raise its transition temperature. PA1 --forming an element of given length with grains of a high temperature superconducting ceramic precursor material extending adjacent each other over all of this length; PA1 --subjecting this element to zone melting over all of its length at a temperature and a speed sufficient to cause the precursor material to be textured into new grains that are aligned and have a good intergranular connectivity; and PA1 --subjecting the so-obtained, melt-textured superconducting ceramic element to oxygen annealing in order to adjust its oxygen level and raise its transition temperature.
As aforesaid, this technique has proven to be efficient at least in labs, to produce in a continuous or semi-continuous manner, short wires of superconducting ceramic, in particular Y Ba.sub.2 Cu.sub.3 O.sub.7-, which are capable of carrying high currents because they have, on the one hand, aligned grains with good intergranular connectivity as a result of the direct solidification achieved by zone melting and, on the other hand, a reduced concentration of impurities over most of their length as a result of the accumulation of these impurities in the melting zone which "moves" along the wire as this wire moves forwards through the tube furnace, and thus moves toward the "rear" end of the wire that can subsequently be cut-off.
In spite of its reported efficiency, the melt-texturing technique has two major limitations which have prevented it from being scaled up so far.
The first one of these major limitations is the slowness of this technique, which calls for processing rates of a few millimeters per hour only to achieve adequate direct solidification. This first limitation has been recognized by R. L. Meng et al who, at the end of their above-mentioned paper, suggest to vary the processing parameters and/or the phase diagram of the ceramic through doping to improve the speed of grain growth.
The other major limitation is the accumulation of the impurities which tend to concentrate in the melt as the process advances and thus to increase along the superconductor that is being formed, until the electrical and mechanical properties of this conductor are ultimately destroyed. This second limitation "limits" to a few centimeters the length of the wires that can be produced according to this technique.